Previously, the preparation of pyrazine and its derivatives has involved the dehydrogenation of the corresponding piperazines, but the method suffers from the disadvantage that the two classes of chemicals are often difficult to separate.
Pyrazines have also been prepared from diols, for example Japanese Patent No. 79,132,588 discloses the preparation of pyrazines by treating diols with diamines in the gas phase in the presence of compounds such as Zn, Mg, Ce, Mn, Fe, Pd, Pb, P or B. In one example 2,3-butanediol and 1,2-diaminopropane were reacted in the presence of a ZnO catalyst containing PdSO.sub.4 at 470.degree.-80.degree. to give a 70% pyrazine product with 100% conversion of the starting materials.
In another Japanese reference, Japanese Patent No. 80,122,769, a method is disclosed for preparing pyrazines from the hydroxydiamine, N-(2-hydroxyethyl)ethylenediamine using a Cr.sub.2 O.sub.3 --CuO catalyst at a temperature from about 265.degree. C. to 300.degree. C. The conversion was about 82% and the yield of pyrazine was about 78%.
Pyrazines have also been prepared from diols plus a diamine through contact in the gas phase in the presence of a zinc-containing catalyst. Such a procedure is disclosed in U.S. Pat. No. 4,097,478.
To our knowledge, the only disclosure of a procedure for making pyrazines from hydroxyamines comes from a recent Japanese Patent Application, No. 60258-168A, which describes subjecting alkanolamines to gaseous reaction conditions in the presence of a zinc-containing catalyst. High temperatures of 300.degree.-500.degree. C. are preferred for this synthesis with this class of catalyst.
The pyrazine products of these syntheses described supra are useful as intermediates for perfumes, pharmaceuticals and agrochemicals. 2,5-Dimethylpyrazine, for example, is useful in cosmetics, flavorings and polymer applications.
It would be a great advantage in the art if pyrazines could be prepared from hydroxyamines under mild reaction conditions. It would also be an advantage if the catalyst system which allowed good conversion and yield also enabled the product to be separated efficiently.